1. Field of the Invention
The present invention relates to a process for producing an optical waveguide, in particular, a flexible polymer optical waveguide.
2. Description of the Related Art
As the process for producing a polymer optical waveguide, the following processes have been proposed: (1) a process comprising impregnating a film with a monomer, exposing a core portion selectively to light to change a refraction index thereof, and sticking a film thereto (selective polymerization); (2) a process comprising applying a core layer and a cladding layer, and forming a cladding portion by reactive ion etching (RIE); (3) a process employing photolithography to perform exposure and development (direct exposure) using an UV-curable resin obtained by adding a photosensitive material to a polymeric material; (4) a process employing injection molding; (5) a process comprising applying a core layer and a cladding layer, and exposing a core portion to light to change a refraction index of the core portion (photo bleaching), or the like.
However, the selective polymerization process (1) has a problem of sticking of the film, and the processes (2) and (3) involve increased costs due to use of photolithography. The process (4) has a problem of poor precision of the resultant core diameter, and the process (5) has a problem of an insufficient refraction index difference between the core layer and the cladding layer.
Currently, practically employable processes exhibiting superior performance are only the processes (2) and (3), however, these processes are associated with the aforementioned problem of increased costs. Any of the processes (1) to (5) cannot be applied to formation of a polymer optical waveguide onto a large and flexible plastic substrate.
In order to produce a polymer optical waveguide, there is known a process comprising filling into a pattern substrate (clad) that has patterned grooves to form capillaries, a polymer precursor material for a core, curing the precursor material to form a core layer, and adhering a flat substrate (clad) onto the core layer. However, this process has a problem in that the polymer precursor material is thinly supplied not only to the capillary groove but also to a space between the pattern substrate and the flat substrate entirely and thereafter cured to form a thin layer having the same composition as the core layer, whereby light leaks out through this thin layer.
As one of the methods of solving this problem, Davit Heard proposed a method comprising fixing and sticking a pattern substrate that has patterned grooves to form capillaries to a flat substrate using a clamping jig, sealing the contact portion between the pattern substrate and the flat substrate with a resin, and reducing the internal pressure to fill the capillaries with a monomer (diallyl isophthalate) solution, to thereby produce a polymer optical waveguide (Japanese Patent gazette No. 3151364). This method uses the monomer as the core forming resin material, instead of any polymer precursor material, to reduce the viscosity of the filling material and fill the capillaries with the filling material by capillarity, to thus prevent the monomer from being introduced into any other member than the capillaries.
However, this method has the following problem: because the monomer is used as the core forming material, the volume shrinkage ratio of the monomer is large when polymerized to form a polymer, and as a result, the transmission loss of the polymer optical waveguide may increase.
This method also has a problem in that due to its complicated procedure, in which the pattern substrate and the flat substrate must be fixed and stuck to each other using the clamp, and fixation at the contact portion must be sealed with the resin, it is impossible to perform mass production using this method and hence cost reduction is unexpected. Moreover, this method cannot be applied to production of the polymer optical waveguide using, as a clad, a film having a thickness in the order of millimeter or a thickness of 1 mm or less.
Recently, George M. Whitesides et al. at Harvard University has proposed a method called capillary micromold as one of soft lithographic processes so as to form a nanostructure. This method comprises forming a master substrate by photolithography, transferring the nanostructure of the master substrate onto a mold made of PDMS utilizing adhesiveness of polydimethylsiloxane (PDMS) and good peeling ability thereof, pouring liquid polymer into this mold by capillarity, and curing the polymer. A detailed review is described in SCIENTIFIC AMERICAN September 2001 (Nikkei Science, 2001, December).
Kim Enoch et al. of George M. Whitesides' group at Harvard University obtained a patent on the capillary micromold method (U.S. Pat. No. 6,355,198).
However, even if the production process disclosed in this patent is applied to production of the polymer optical waveguide, it takes a prolonged time to form its core portion since the sectional area of the core portion of the polymer optical waveguide is very small, thus making the process unsuitable for mass production. This process also has a drawback in that when a monomer solution is polymerized to form a polymer, a volume change occurs to alter the shape of the core, whereby the transmission loss increases.
B. Michel et al. at IBM Zurich Laboratory proposed a lithographic technique using PDMS, and reported that this technique achieved high resolution in the order of several tens of nanometers. A detailed review is descried in IBM J. REV. & DEV., Vol. 45 No. 5, Sep. 2001.
As described above, the soft lithographic technique using PDMS and the capillary micromold method are the focus of recent attention as nanotechnology in the United States and some other countries.
However, even when the optical waveguide is formed by a micromold method, it is impossible to simultaneously fulfil the requirements of reduced volume shrinkage ratio (reduction of transmission loss) occurred when curing and lowered viscosity of a filling liquid (the monomer, etc.) to facilitate the filling. Accordingly, if reduced transmission loss is preferentially considered, the viscosity of the filling liquid cannot be lowered to a level below a specified limit, whereby the filling speed decreases, and hence the mass production of optical waveguides cannot be expected. When the micromold method is carried out, it requires use of a glass or silicon plate as the substrate, and thus use of a flexible film substrate is not considered.
Under these circumstances, it can be considered that a method of forming a flexible polymer optical waveguide in which an optical waveguide is provided on a film substrate. This method involves simplified producing steps and allows easily production of the polymer optical waveguide at considerably reduced costs, as compared to conventional methods for producing a polymer optical waveguide.
Such a flexible polymer optical waveguide is required to have a function to allow connection to plural optical parts. Since a light emitting portion of a light emitting element and a light receiving portion of a light receiving element are different from each other in an area and a shape, it is desirable to arbitrarily change a sectional area or a sectional shape of input and output portions of the optical waveguide to achieve connection. In order to meet such a requirement, for example, a method to prepare the optical waveguide itself using an RIE process may be used.
As a process for producing a (non-flexible) polymer optical waveguide which allows connection to another optical part such as an optical fiber, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-253845 discloses a process by which an optical fiber is connected to a polymer optical waveguide using photolithography. However, this process requires a patterning exposure every time, because of photolithography, when an optical waveguide is formed. In order to connect the polymer optical waveguide to the optical fiber, procedures comprising configuring a shallow liquid-collecting pool in a substrate and grooves at both sides of the pool and placing the optical fiber in the thus formed groove is adopted in this process. In such procedures, it is necessary to work not only the liquid-collecting pool but also the grooves in the substrate, to thereby increase the number of the steps. Moreover, it is necessary for each of polymer optical waveguides to be formed such that the position of the grooves is precisely matched to the photo mask. For the foregoing reasons, a problem of increased cost still remains. In order to three-dimensionally change the sectional area of the optical waveguide in the longitudinal direction, it is necessary to precisely control the depth of the shallow liquid-collecting pool, which poses another problem of a reduced yield.